1. Field of the Invention
This invention is directed to adhesives, especially denture adhesives, and to methods of making and using such adhesives.
2. Description of Related Art
Dentures are usually secured in the mouth using creams or powders that have adhesive properties. These denture adhesive compositions serve both to adhere the dentures to the gums and also to provide a cushion and sealant between the gums and the dentures to thereby permit a comfortable fit. The denture adhesives are also required to have acceptable organoleptic qualities. They further need to be sufficiently strong so that a single application of the adhesive is sufficient for a full day's use while not preventing or hindering removal of the dentures at the end of the day for cleaning and the like. The latter requirement has taken on more prominence in recent years since the number of hours of continuous wear which is considered to constitute a "full day" has been increasing.
The technical literature discloses a wide variety of denture adhesive materials. However, most presently available commercial denture adhesives are based on all vinyl ether/maleic acid or anhydride copolymers. This class of adhesives was set forth as a potential denture adhesive in Germann et al., U.S. Pat. No. 3,003,988, which issued more than 30 years ago. That patent describes synthetic, water sensitized but water insoluble, materials comprising mixed partial salts of lower alkyl vinyl ether-maleic anhydride copolymers for stabilizing dentures. The salts mentioned in the patent are a mixture of (a) calcium and (b) alkalies including sodium, potassium and quaternary ammonium compounds in a 1:1 to 5:1 molar ratio. The calcium and alkali materials are added to the copolymer to form a mixed salt. The use of this class of materials has been described in a variety of other patents. Examples include U.S. Pat. No. 4,989,391, 5,037,924, 5,093,387, 4,980,391 and 4,373,036, European Published Patent Application No. 406,643 and WO 92/10988.
These polymers and their salts have not proven to be as fully effective as desired for securing the denture, particularly when the denture wearer consumes hot liquids. Additionally, they have been found susceptible to "washout", a term which refers to the erosion of the adhesive from under the denture during use. As a result, the formulations have provided less than the 12 hours of holding power that denture wearers have begun to demand.
In order to provide additional adhesive and cohesive properties, one approach which has been taken is to manipulate the salt form of the copolymer. Examples can be found in WO 92/22280, WO 92/10988, WO 92/10987 and U.S. Pat. No. 4,758,630 and 5,073,604. Another approach has been to employ an adhesion adjuvant in the formulation or converting the copolymer into a terpolymer, and examples of these approaches can be found in U.S. Pat. No. 3,736,274, 5,037,924 and 5,093,387.
Despite the efforts that have been put into improving the properties of maleic anhydride/alkyl vinyl ether type polymers and their salts, these formulations do not provide the full desired measure of adhesion, cohesion and resistance to washout from beneath the denture.
The copolymer can be made in a variety of molecular weights and an appropriate value is selected depending on the use to which it is going to be put. It is generally recognized that the copolymer of the alkyl vinyl ether and maleic anhydride must have a suitably high molecular weight in order to be suitable for use as a denture adhesive. The minimum molecular weight is frequently indicated as corresponding to a specific viscosity of about 1.2 when measured in a 1% weight/volume solution in methyl ethyl ketone at 25.degree. C.
How high the molecular weight can be in excess of that value is rarely mentioned or considered to be important and the description in WO 92/10988 is a good example of this fact. U.S. Pat. No. 4,373,036 is a rare exception in that it indicates the specific viscosity can be about 1.5-3.5.
The dearth of consideration of molecular weight is understandable both on a theoretical and experimental basis. As molecular weight increases, the solubility of the polymer decreases. The polymer develops its adhesive strength when it is hydrated by the moisture in an individual's saliva and decreased solubility would make the polymer harder to hydrate. Accordingly, the expectation is that as the molecular weight increases, the solubility would decrease to the point where the polymer would no longer function suitably as an adhesive.
On an experimental basis, it has been observed that a copolymer having a specific viscosity of about 3.5 does not have any statistically significant adhesive properties compared to those of a copolymer having a specific viscosity of about 2.6. Since it is more difficult to produce the higher molecular weight polymer, there is no reason to do so. In view of these considerations, it is not surprising that attempts to improve denture adhesive compositions have ignored the molecular weight the polymer. Surprisingly, it has now been found that when the molecular weight is sufficiently high, a superior denture adhesive is achieved.
It is the object of this invention to provide an improved denture adhesive composition having both strong adhesive properties and good washout resistance to provide long holding properties. This and other objects of the invention will become apparent to those of ordinary skill in this art from the following detailed description.